Effects of two and a half years of atmospheric CO2 enrichment on the root density distribution of three-year-old sour orange trees

Agricultural and Forest Meteorology, 55 ( 1991 ) 345-349

345

Elsevier Science Publishers B.V., Amsterdam

Short Communication

Effects of two and a half years of atmospheric C O 2 enrichment on the root density distribution of three-year-old sour orange trees* S.B. Idso and B.A. Kimball U.S. Water Conservation Laboratory, 4331 E. Broadway, Phoenix, AZ 85040, USA (Received 29 September 1990; revision accepted 25 November 1990)

ABSTRACT Idso, S.B. and Kimball, B.A., 1991. Effects of two and a half years of atmospheric CO2 enrichment on the root density distribution of three-year-old sour orange trees. Agric. For. Meteorol., 55: 345-349. Eight sour orange trees planted directly into the ground at Phoenix, Arizona, as small seedlings in July 1987 have been enclosed by four clear-plastic-wall, open-top chambers since November of that year, half of which have been continuously supplied with a CO2 enriched atmosphere consisting of an extra 300 cm 3 CO2 m-3 of air. Extensive soil coring of the trees' root zones conducted in July 1990 indicated that two and a half years of growth under these conditions produced a fine root biomass enhancement of 175% in the CO2 enriched trees. This growth enhancement is of the same order of magnitude as our previously reported results for net photosynthesis and trunk and branch volumes for these trees.

INTRODUCTION

In July 1987, we transplanted eight 30-cm tall sour orange tree (Citrus aurantium) seedlings directly into the ground at Phoenix, Arizona. Since November of that year, half of the trees (which are enclosed in pairs by clearplastic-wall open-top chambers) have been maintained at the ambient CO2 concentration of the local environment, while half of them have been continuously supplied with air enriched to 300 cm 3 CO2 m -3 air above ambient. In the two reports of this continuing study that have been published to date, we documented a CO2 induced increase in above-ground biomass of 179% after two complete years of CO2 enrichment (Idso et al., 1991 a), which appears to *Contribution from the Agricultural Research Service, US. Department of Agriculture. Supported in part by the Institute for Biospheric Research and the U.S. Department of Energy, Carbon Dioxide Research Division, Office of Energy Research, under Interagency Agreement No. DE-AI05-88ER-69014.

0168-1923/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

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S.B. IDSO AND B.A. KIMBALL

be the result of an equivalent increase in net photosynthesis rate, as inferred from measurements of this parameter carried out over the second growing season of the experiment (Idso et al., 1991b). In this paper, we begin our analysis of the below-ground consequences of atmospheric CO2 enrichment, reporting the results of a detailed study of the root growth of the trees after two and a half years of differential CO2 treatment. MATERIALS AND METHODS

Soil cores for root analyses were extracted during July 1990 at specified intervals on four transects running outward from each tree trunk in NE, NW, SE and SW directions. The sampling positions corresponded to (B) the mean canopy edge of each tree, (A) the midway point between this location and the tree's trunk, ( D ) twice the canopy edge distance from the trunk, and (C) midway between this location and the canopy edge. At each of these 16 positions about each tree, soil cores 41 mm in diameter were obtained from depth intervals of 0-20, 20-40, 40-60, 60-80, 80-100 and 100-1200 cm, making a total of 96 soil cores per tree or 768 for the entire study. This sampling procedure required 4 weeks to complete. Hence, each day's set of cores was stored in a walk-in freezer set at - 15 ° C until all samples were obtained. Each day thereafter, for another 4 weeks, one day's worth of samples were removed from the freezer and allowed to thaw overnight. The following day, these samples were flushed through a set of sieves, the smallest TABLE 1 Fine root biomass density distributions (dry weight of roots per m 2 of surface ground area ) Soil depth (cm)

0 - 20 2 0 - 40 40- 60 6 0 - 80 80-100 100-120

Ambient

Enriched

A

B

C

D

A

B

C

D

71.4 _+15.8 46.5 _+ 1.5 42.8 _+25.8 5.8 + 1.6 1.8 -+ 1.1 1.9 -+ 1.8

39.5 _+ 8.7 31.6 _+ 2.3 38.8 _+ 5.9 2.8 _+ 0.4 1.9 -+ 1.6 3.4 _+ 0.6

26.1 _+12.6 29.9 _+ 4.8 5.9 _+ 0.4 0.8 _+ 0.2 3.7 -+ 3.7 0.8 -+ 0.9

12.5 _+ 9.6 17.4 -+ 1.8 3.5 _+ 0.2 0.4 _+ 0.2 2.3 -+ 3.0 0.2 -+ 0.2

128.7 _+ 51.7 86.9 -+ 13.6 40.3 _+ 7.0 14.6 _+ 5.4 3.5 _+ 0.1 6.4 _+ 2.7

73.6 +17.4 54.1 _+ 6.4 26.0 _+ 8.0 5.2 -+ 2.8 1.5 _+ 0.1 5.0 -+ 6.9

37.4 + 2.6 56.4 _+ 7.6 16.1 _+16.0 3.1 _+ 3.4 0.4 _+ 0.1 0.7 ± 0.1

20.0 +_ 3.0 42.2 _+ 0.0 16.3 _+ 5.0 2.0 _+ 0.3 0.4 _+ 0.4 1.3 + 1.1

The letters A, B, C and D, respectively, represent mean radial distances from the tree trunks o f 35, 70, 105 and 140 cm, in the case of ambient trees, and distances o f 40, 80, 120 and 160 cm, in the case of enriched trees. Figures in the table represent means and their associated standard deviations.

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E F F E C T O F A T M O S P H E R I C CO2 E N R I C H M E N T O N R O O T D E N S I T Y D I S T R I B U T I O N

mesh size of which was just under 1 mm. The roots collected by these means were separated into two groups: fine, consisting of the smallest root subdivision generally observed, which had a mean diameter of 0.6 mm, and large, consisting of everything larger. These collections were dried for 2 days in an oven set at 55 °C and then weighed to determine the amount of dry matter they contained. RESULTS

Fine roots were by far the most predominant type encountered. In fact, large roots were so few and far between, and so variable in size, that we could not obtain meaningful characterizations of their spatial distributions. Neither 120

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Fig. 1. Fine root biomass density vs. distance from tree trunk for the soil depth interval 40-120 cm (top) and the 0-40 cm layer (bottom).

348

S.B. IDSOAND B.A. KIMBALL

TABLE 2 Data and calculations relative to the fine root biomass of the ambient (Amb) and CO2 enriched (Enr) trees Radial distance (cm)

0- 10 10- 20 20- 30 30- 40 40- 50 50- 60 60- 70 70- 80 80- 90 90-100 100-110 110-120 120-130 130-140 140-150 150-160 160-170 170-180 180-190 190-200 200-210 210-220 220-230 230-240 240-250 250-260

Area (m 2)

0.0314 0.0942 0.1571 0.2199 0.2827 0.3456 0.4084 0.4712 0.5341 0.5969 0.6597 0.7226 0.7854 0.8482 0.9111 0.9739 1.0367 1.0996 1.1624 1.2252 1.2881 1.3509 1.4137 1.4766 1.5394 1.6022

g root m-2

g root

Cumulative g root

Amb

Enr

Amb

Enr

Amb

Enr

213.8 199.0 184.0 170.2 155.0 140.2 125.8 110.2 95.8 81.0 67.2 58.0 49.0 40.0 31.2 21.2 13.5 4.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

381.2 352.0 323.8 295.0 267.0 237.8 209.0 180.2 159.0 146.0 132.3 119.6 110.0 101.8 94.0 85.5 78.2 70.0 61.2 53.6 46.7 37.8 29.7 21.3 13.6 5.2

6.7 18.7 28.9 37.4 43.8 48.5 51.4 51.9 51.2 48.3 44.3 41.9 38.5 33.9 28.4 20.6 14.0 5.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

12.0 33.2 50.9 64.9 75.5 82.2 85.4 84.9 84.9 87.1 87.3 86.4 86.4 86.3 85.6 83.3 81.1 77.0 71.1 65.7 60.2 51.1 42.0 31.5 20.9 8.3

7 25 54 92 136 184 235 287 338 387 431 473 512 545 574 594 608 614 614 614 614 614 614 614 614 614

12 45 96 161 236 319 404 489 574 661 748 835 921 1007 1093 1176 1257 1334 1406 1471 1531 1582 1624 1656 1677 1685

En.__£r Amb

Cumulative percentage enhancement

1.71 1.80 1.78 1.75 1.74 1.73 1.72 1.70 1.70 1.71 1.74 1.77 1.80 1.85 1.90 1.98 2.07 2.17 2,29 2,40 2,49 2,58 2.64 2.70 2,73 2.75

71 80 78 75 74 73 72 70 70 71 74 77 80 85 90 98 107 117 129 140 149 158 164 170 173 175

could we accurately determine the large root biomass differential between treatments. Hence, the analyses reported here pertain solely to fine roots. Table 1 summarizes our findings by soil depth and radial distance from the trunk for the ambient and CO2 enriched trees. In addition, we have plotted the combined results for the bottom four layers and top two soil layers in Fig. 1. As can be seen, CO2 effects are not significant from 40 to 120 cm depth. The 0-40 cm layer does show significant CO2 effects, however, and as 70% of the ambient trees' roots and 78% of the CO2 enriched trees' roots are located within this depth interval, the implications for total root growth are considerable. If we sum the results for all six soil layers, for example, and extrapolate to the trunk and the radial distances at which the root densities go to zero (as in the lower portion of Fig. 1 ), we can calculate the total fine root biomass of

EFFECT OF ATMOSPHERIC CO2 ENRICHMENT ON ROOT DENSITY DISTRIBUTION

349

each treatment as per the procedure recorded in Table 2. As indicated there, although the fine root biomass density is enhanced by approximately 75% beneath the trees' canopies, the fact that the roots of the CO2 enriched trees extend farther out from their trunks than do the roots of the ambient trees results in a total biomass enhancement of 175%. DISCUSSION

A 175% enhancement of fine root biomass produced by a 300 cm 3 CO2 m - 3 enrichment of the air may seem inordinately large, but other measurements we have made on the trees would appear to confirm its reality. Idso et al. (1991a), for example, found the CO2 induced enhancement of total aboveground trunk plus branch volume to be 179%. Idso et al. (1991b) also found the CO2 induced enhancement of net photosynthesis to range from 50% in the early morning hours to > 300% in the afternoon. Perhaps one reason for this dramatic growth response to atmospheric CO2 enrichment is that sour orange trees do not appear to close their stomates much as the CO2 content of the air is raised (Idso, 1991 ). Hence, they are able to take greater advantage of the increased air-to-leaf CO2 gradient provided by the CO2 enrichment than are most agricultural crops, which generally experience a significant concomitant reduction in stomatal conductance. Another possible cause of the large growth stimulation may be the high temperatures experienced at Phoenix. As Idso et al. ( 1987 ) and Allen et al. (1990) have shown, the aerial fertilization effect of CO2 rises significantly with increasing air temperature.

REFERENCES Allen, S.G., Idso, S.B., Kimball, B.A., Baker, J.T., Allen, Jr., L.H., Mauney, J.R., Radin, J.W. and Anderson, M.G., 1990. Effects of air temperature on atmospheric COz-plant growth relationships. U.S. Department of Energy, Washington, DC, 61 pp. Idso, S.B., 1991. A general relationship between CO2-induced increases in net photosynthesis and concomitant reductions in stomatal conductance. Exp. Environ. Bot., submitted. Idso, S.B., Kimball, B.A., Anderson, M.G. and Mauney, J.R., 1987. Effects of atmospheric CO2 enrichment on plant growth: the interactive role of air temperature. Agric. Ecosystems Environ., 20: 1-10. Idso, S.B., Kimball, B.A. and Allen, S.G., 1991 a. CO2 enrichment of sour orange trees: two-anda-half years into a long-term experiment. Plant Cell Environ., in press. ldso, S.B., Kimball, B.A. and Allen, S.G., 199 lb. Net photosynthesis of sour orange trees maintained in atmospheres of ambient and elevated CO2 concentration. Agric. For. Meteorol., 54: 95-101.